Handover method and apparatus in a wireless communication system including femto cells

ABSTRACT

A handover method and apparatus for facilitating a handover of a user equipment to a femto cell in a wireless communication system supporting both the femto and macro cells. When a handover condition is detected, the user equipment acquires identity information of a target femto cell base station from system information transmitted by the target femto cell base station, decides whether to handover to the target femto cell base station, based on the identity information; and transmits a measurement report message including the identity information to a serving base station of the user equipment, when the handover is to be made.

PRIORITY

This application is a Continuation Application of U.S. patentapplication Ser. No. 12/705,278, filed in the U.S. Patent and TrademarkOffice on Feb. 12, 2010, and claims priority under 35 U.S.C. §119(a) toan application filed in the Korean Intellectual Property Office on Feb.13, 2009 and assigned Serial No. 10-2009-0011891, the content of each ofwhich is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to wireless communication and,in particular, to a handover method and apparatus for facilitating ahandover to a femto cell in a wireless communication system includingfemto and macro cells.

2. Description of the Related Art

A femto cell is a small cellular base station designed to provideshort-range coverage of about 10 meters for use in a home or smallbusiness and a broadband connection to a core network. Commonly, a femtocell extends the cellular network coverage indoors, providing users withhigher quality and fixed-mobile convergence services.

The 3^(rd) Generation Partnership Project (3GPP) considers anenvironment in which Closed Subscriber Group (CSG) cells and legacymacro cells coexist. Typically, the terms “CSG cell”, “femto cell”, “CSGcell base station”, and “femto cell base station” are synonymously used.In a 3GPP Long Term Evolution (LTE) system, an evolved Node B (eNB)denotes the macro cell base station, and a Home eNB (HeNB) denotes afemto cell base station (or CSG cell base station).

The network architecture designed to support a femto cell enablesdeployment of a plurality of femto cell base stations and a UserEquipment (UE) to measure the signals of the femto cell and macro cellbase stations (or eNBs). For a handover between the macro cell eNBs, theUE analyzes a signal transmitted by a handover target eNB to acquire aPhysical Cell ID (PCI) and sends a measurement report based on the PCIto a currently serving eNB such that the serving eNB can make a handoverdecision.

For a handover from a macro cell eNB to a femto cell eNB (i.e., HeNB) orfrom an HeNB to another HeNB, however, the network must also determinewhether the particular UE is permitted to access the target HeNB.Accordingly, there is a need to develop a method for facilitating ahandover of a UE to a femto cell (CSG cell or HeNB) in a wirelesscommunication system supporting deployment of femto cells.

SUMMARY OF THE INVENTION

The present invention provides a handover method and apparatus capableof facilitating a handover to a femto cell, considering the mobility ofthe UE, in a wireless communication system supporting both the femto andmacro cells.

In accordance with an embodiment of the present invention, a measurementreport method for a handover of a User Equipment (UE) in a wirelesscommunication system including femto cells is provided. The methodincludes detecting, by the UE, a handover condition; acquiring, by theUE, identity information of a target femto cell Base Station (BS) fromsystem information transmitted by the target femto cell BS; determining,by the UE, whether to handover to the target femto cell BS, based on theidentity information; and transmitting, by the UE, a measurement reportmessage including the identity information to a serving BS of the US,when the UE determines to handover to the target femto cell BS.

In accordance with another embodiment of the present invention, ahandover apparatus for a wireless communication system including femtocells is provided. The apparatus includes a storage unit that stores anaccess allowed list including identity information of a femto cell BaseStation (BS) permitting access by a User Equipment (UE); a femto cellidentity information extractor that retrieves identity information of atarget femto cell BS from the access allowed list, when a handover isrequired; an access allowed femto cell verifier that verifies the targetfemto cell BS by receiving identity information of the target femto cellBS and comparing the received identity information with the identityinformation retrieved from the access allowed list; and a measurementmessage generator that generates a measurement message and sends themeasurement message to a serving BS of the UE, when the target femtocell is verified.

In accordance with another embodiment of the present invention, a methodfor verifying a femto cell Base Station (BS) to which a User Equipment(UE) attempts a handover in a wireless communication system includingfemto cells is provided. The method includes receiving, by the femtocell BS, a handover request message including identity information of atarget femto cell BS for the handover from a mobility management entity;comparing, by the femto cell BS, the identity information of the targetfemto cell BS with a current identity information of the femto BS todetermine whether the identity information correspond with each other;and accepting, by the femto cell BS, the handover of the UE to the femtocell BS, when the identity information correspond with each other.

In accordance with another embodiment of the present invention, a femtocell Base Station (BS) of a wireless communication system including aplurality of femto and macro cells is provided. The femto cell BSincludes a Radio Frequency (RF) unit that receives a handover requestmessage including identity information of a target femto cell BS, thehandover request message being transmitted by a mobility managemententity; a storage unit that stores cell information of the femto cellBS; a control unit that extracts the identity information of the targetfemto cell BS from the handover request message; and a cell identityverifier that verifies that the femto cell BS is the target femto cellBS by comparing the received identity information of the target femtocell BS with current identity information of the femto cell BS, and thataccepts the handover of the UE to the femto cell BS, when the receivedidentity information of the target femto cell BS corresponds with thecurrent identity information of the femto cell BS.

In accordance with still another embodiment of the present invention, ahandover method of a user equipment in a wireless communication systemincluding a plurality of femto cells is provided. The method includesdetermining, by the UE, that a handover condition is satisfied; sendinga measurement report message including identity information of a targetfemto cell Base Station (BS) from the UE to a serving BS; determining,at the serving BS, a handover to the target femto cell BS; sending ahandover required message including the identity information of thetarget femto cell BS from the serving BS to a mobility managemententity; determining, by the mobility management entity, whether tohandover the UE to the target femto cell BS; sending a handover requestmessage including the identity information of the target femto cell BSfrom the mobility management entity to the target femto cell BS, whenthe mobility management entity determines to handover the UE; comparing,at the target femto cell BS, the identity information included in thehandover request message and current identity information of the targetfemto cell BS; and accepting the handover of the UE to the target femtocell BS, by the target femto cell BS, when the identity informationincluded in the handover request message and current identityinformation of the target femto cell BS correspond with each other.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features, and advantages of the presentinvention will be more apparent from the following detailed descriptionin conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating a wireless communicationnetwork according to an embodiment of the present invention;

FIG. 2 is a block diagram illustrating a UE according to an embodimentof the present invention;

FIG. 3 is a flowchart illustrating a handover method of a UE in awireless communication system according to an embodiment of the presentinvention;

FIG. 4 is a sequence diagram illustrating operations of network entitieswhen a UE performs a handover to a femto cell base station according toan embodiment of the present invention;

FIG. 5 is a sequence diagram illustrating operations of network entitieswhen a UE performs a handover to a femto cell base station according toan embodiment of the present invention;

FIG. 6 is a flowchart illustrating operations of a target femto cellbase station in the handover procedure illustrated in FIG. 5;

FIG. 7 is a sequence diagram illustrating operations of network entitieswhen a UE performs a handover to a femto cell base station according toan embodiment of the present invention;

FIG. 8 is a sequence diagram illustrating operations of network entitieswhen a UE performs a handover to a femto cell base station according toan embodiment of the present invention; and

FIG. 9 is a block diagram illustrating a configuration of a femto cellbase station for supporting handover procedures according to anembodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Various embodiments of the present invention are described in detailbelow with reference to the accompanying drawings. The same referencenumbers are used throughout the drawings to refer to the same or likeparts. Further, detailed descriptions of well-known functions andstructures incorporated herein may be omitted to avoid obscuring thesubject matter of the present invention.

Although the embodiments of the present invention will be describedbelow using terms defined in the 3GPP LTE standards, the presentinvention is not limited thereto.

Additionally, unless otherwise noted, the terms “cell”, “base station”,and “node B” are used interchangeably herein. Also, the terms “macrocell”, “macro cell base station”, and eNB” are used interchangeably; andthe term “femto cell”, “femto cell base station”, “femto cell eNB”, and“HeNB” are used interchangeably.

FIG. 1 is a schematic diagram illustrating a wireless communicationnetwork according to an embodiment of the present invention.

Referring to FIG. 1, the wireless communication network includes a macrocell base station (eNB) 110, a femto cell base station (HeNB) 120, aMobility Management Entity (MME) 130, a HeNB Gateway (HeNB GW) 140, anda UE 150. Additionally, the HeNB GW 140 can be omitted depending on thesystem implementation.

The eNB 110 is a base station responsible for managing a macro cell. Themacro cell is a cell of a typical cellular system and the eNB 110 is thebase station managing the macro cell defined by its radio coverage. Asindicated above, the terms “macro cell” and “eNB” are usedinterchangeably herein.

The eNB 110 establishes a wireless channel with the UE 150 and controlsradio resources. For example, the eNB 110 broadcasts system informationincluding control information required for the UE 150 to access the eNB110 and allocates radio resources to the UE 150 for exchanging data andcontrol information. The eNB 110 also collects the channel measurementsof a serving cell and neighbor cells that are transmitted by the UE 150,makes a handover determination, and instructs the UE 150 to perform thehandover. For this purpose, the eNB 110 is provided with a controlprotocol such as Radio Resource Protocol (RRC) related to the radioresource management.

The HeNB 120 connects to a core network via a base station controller(not shown) and the HeNB GW 140 to provide the UE 150 with wirelesscommunication services. The HeNB 120 often referred to as a “ClosedSubscriber Group (CSG) cell,” which permits a specific group of UEs toaccess a femto cell. The CSG cell is a cellular network deployed in ahome, small business, or a specific place to provide an access serviceto a specific organization. In accordance with an embodiment of thepresent invention, the HeNB 120 only permits access to previouslyregistered UEs and broadcasts a group identity information, i.e., a CSGID, in a System Information Block (SIB).

The MME 130 is manages UEs in an idle mode and selects a Packet DataNetwork (PDN) and Service Gateway (not shown). The MME 130 also isresponsible for the roaming and authentication of the UEs.

In accordance with an embodiment of the present invention, the MME 130stores identity information about femto cell base stations permittingaccess of a UE in a UE-specific allowed CSG list. The MME 130 can sendthe UE-specific allowed CSG list to the serving eNB 110 and the UE 150in separate messages.

The HeNB GW 140 is responsible for connecting the HeNB 120 to the MME130. As indicated above, the HeNB GW 140 can also be omitted dependingon the system implementation.

The UE 150 can connect to the core network via the eNB 110 or the HeNB120. The UE 150 receives the UE-specific allowed CSG list from the MME130. The UE 130 acquires identity information of the femto cells (i.e.,HeNBs) from the SIB broadcasted by the HeNB 120 and identifies the HeNB120 by matching the acquired identity information to a CSG ID in theUE-specific allowed CSG list.

The UE 150 can receive the femto cell-specific Physical Cell ID from theeNB 110 or the HeNB 120 and search for the Synchronization Channel (SCH)and perform channel measurements using the femto cell-specific physicalCell ID.

As described above, the wireless communication network according to anembodiment of the present invention is assumed as a cellular systemsupporting both the eNBs and HeNBs. Although a description will beprovided below in which the service base station is an eNB, the presentinvention is also applicable when the serving base station is an HeNB.

In the description below, a serving base station providing a UE withmobile communication services has information about neighbor femto cellsin a Neighbor Relation Table (NRT). The NRT of the serving base stationstores the Physical Cell ID (PCI), an Evolved Universal MobileTelecommunications System (UMTS) Terrestrial Radio Access Network(EUTRAN) Cell Global ID (ECGI), a Tracking Area ID (TAI), a CSGindicator indicating whether the base station is a CSG base station, andinformation about the CSC ID.

FIG. 2 is a block diagram illustrating a UE according to an embodimentof the present invention.

Referring to FIG. 2, the UE includes a Radio Frequency (RF) unit 210, astorage unit 220, and a control unit 230. The storage unit 220 includesan allowed CSG list storage region 220A. The control unit 230 includes afemto cell identity information extractor 230A, an access allowed femtocell determiner 230B, and a measurement report message generator 230C.

The RF unit 210 includes an RF transceiver and a duplexer. The RF unit210 is responsible for processing radio signals transmitted and receivedby the UE.

The storage unit 220 stores programs and data related to operations ofthe UE. In accordance with an embodiment of the present invention, thestorage unit 220 includes the allowed CSG list storage region 220A. Theallowed CSG list includes identity information of femto cell basestation to which the UE 150 is permitted to access with the CSG IDs ofthe access-allowed femto cell base stations. As described above, inaccordance with an embodiment of the present invention, the allowed CSGlist is transmitted by the MME 130.

The control unit 230 controls the overall operations of the UE 150.Particularly, the control unit 230 can include a data processing unit(not shown) having a transmission data processor and a reception dataprocessor.

The transmission data processor can include a coder for encoding asignal to be transmitted, a modulator for modulating the encoded signal,and a digital/analog converter for converting the modulated signal to ananalog signal. The coder can include a data coder for processing packetdata and an audio coder for processing audio signal including voice.

The reception data processor can include an analog/digital converter forconverting the analog signal received by the RF unit 210 to a digitalsignal, a demodulator for demodulating the digital signal output by theanalog/digital converter, and a decoder for decoding the demodulatedsignal. The decoder can include a data decoder for processing the packetdata and an audio decoder for processing the audio signal includingvoice.

Further, the control unit 230 includes a femto cell identity informationextractor 230A and an access-allowed femto cell determiner 230B.

When a handover to a femto cell is required, the femto cell identityinformation extractor 230A receives system information transmitted by atarget femto cell base station and extracts identity information of thetarget femto cell base station form the system information. Inaccordance with an embodiment of the present invention, the systeminformation includes a Master Information Block (MIB) or an SIB.Although a description is provided below with the system informationbeing the SIB, the present invention is not limited thereto.

In accordance with an embodiment of the present invention, the identityinformation of the target femto cell base station can be an ECGI.Accordingly, although a description is provided below with the identityinformation being the CSG ID, the present invention is not limitedthereto.

The femto cell identity information extractor 230A transfers theextracted identity information of the target femto cell base station tothe access-allowed femto cell determiner 230B.

The access-allowed femto cell determiner 230B receives the identityinformation of the target femto cell base station from the femto cellidentity information extractor 230A and compares the identityinformation of the target femto cell base station to IDs stored in theaccess-allowed CSG list to determine whether the target femto cell basestation is an access-allowed femto cell base station.

When the target femto cell base station is an access-allowed femto cellbase station, the measurement report message generator 230C generates ameasurement report including the identity information received from thetarget femto cell base station. The measurement report message generator230C transmits the generated measure report message to the serving basestation.

FIG. 3 is a flowchart illustrating a handover method of a UE in awireless communication system according to an embodiment of the presentinvention.

Referring to FIG. 3, when a handover is required, the UE 150 detects atarget femto cell base station in step S310. In step S320, the UE 150reports the detection of the target femto cell base station to theserving base station and receives a target femto cell detection commandfrom the serving base station. In step S330, the UE 150 receives an SIBbroadcasted by the target femto cell base station and acquires identityinformation of the target femto cell base station from the SIB. Here,the identity information of the femto cell base station is a CSG ID.

In step S340, the UE 150 compares the identity information of the targetfemto cell base station with a femto cell list included in anaccess-allowed list to determine whether the target femto cell is anaccess-allowed femto cell. If the target femto cell is an access-allowedfemto cell, the UE 150 generates a measurement report message includingthe acquired identity information (CSG ID) and sends the measurementreport message to the serving base station in step S360. However, if thetarget femto cell is not an access-allowed femto cell, the UE 150ignores the detected femto cell in step S350.

After sending the measurement report message to the serving base stationin step S360, the UE 150 waits to receive a handover command in stepS370. When a handover command has been received, the UE 50 performs thehandover to the detected femto cell base station in step S380.

FIG. 4 is a sequence diagram illustrating operations of network entitieswhen a UE performs a handover to a femto cell base station according toan embodiment of the present invention. More specifically, in theembodiment illustrated in FIG. 4, information on a target femto cellbase station is included in a Neighbor Relation Table (NRT).

Referring to FIG. 4, a serving base station 420 stores information aboutneighbor femto cell base stations (such as PCI, CSG ID, ECGI, and TAO ofeach neighbor femto cell base station) in an NRT. The serving basestation 420 receives an access allowed list from an MME 430 and storesthe access allowed list in a storage unit therein.

The serving base station 420 first receives the ECGI of the target femtocell base station 450 in a Measurement Report message transmitted by theUE 150 in step S405. The ECGI is an identifier mapped to a femto cellbase station for uniquely identifying the femto cell base station. Theserving base station 420 stores the ECGI within the NRT.

After the Measurement Report message has been received, the serving basestation 420 checks whether a CSG ID of a target femto cell is includedin the allowed CSG list of the UE 150 and if it is included, makes adecision to handover the UE 150 to the target femto cell base station450 in step S410. In step S415, the serving base station 420 sends aHandover Required message to the source MME 430.

Alternatively, when the serving base station 420 does not include anallowed CSG list, step S410 is omitted, and the source MME 430 is ablemake a handover decision, after it receives the Handover Requiredmessage.

When required, the source MME 430 can send a Forward Relocation Requestmessage to a target MME 440 to which the target femto cell base station450 is connected in step S420. However, when the target femto cell basestation 450 is managed by the source MME 430, there is no need totransmit the Forward Relocation message.

In step S425, the target MME 440 sends a Handover Request message to thetarget femto cell base station 450.

Upon receipt of the Handover Request message, the target femto cell basestation 450 sends a Handover Request Acknowledgement (ACK) message tothe target MME 440 in step S430.

In step S435, the target MME 440 sends a Forward Relocation Responsemessage to the source MME 430, and in step S440, the source MME sends aHandover command to the serving cell 420.

Upon receipt of the Handover command, the serving cell 420 forwards theHandover command to the UE 150 in step S445.

In step S450, the UE 150 performs the handover to the target femto cellbase station 450.

FIG. 5 is a sequence diagram illustrating operations of network entitieswhen a UE performs a handover to a femto cell base station according toan embodiment of the present invention. More specifically, in theembodiment illustrated in FIG. 5, information on a target femto cellbase station is not included in an NRT of a serving base station.

Referring to FIG. 5, the serving base station 420 does not store theinformation about the neighbor femto cell base stations in an NRT.Instead, the serving base station 420 receives an allowed CSG list fromthe source MME 130 and stores the allowed CSG list. The allowed CSG listincludes information about neighbor femto cells that allow access of theUE 150.

In step S501, when a handover is required, the UE 150 receives an SIBbroadcast by a target femto cell base station 450. The UE 150 canreceive the SIB in response to a command transmitted by the serving basestation 420 or according to its own decision.

The UE 150 acquires identity information of the target femto cell basestation 450 from the received SIB; for example, the identity informationof the target femto cell base station 450 being the CSG ID used by thetarget femto cell base station 450. Thereafter, the UE 150 compares theidentity information of the target femto cell base station with valuesincluded in the allowed CSG list to determine whether the target femtocell base station 450 allows access by the UE 150. If the target femtocell base station 450 is an access allowed femto cell base station, theUE 150 sends a Measurement Report message including the identityinformation of the target femto cell base station 450, ECGI, and TAI tothe serving base station 420 in step 505.

Alternatively, the identity information of the target femto cell basestation 450 can be sent by some other message than a Measurement Reportmessage.

After receiving the Measurement Report message, the serving base station420 checks whether the identity information of the target femto cellbase station 450 is included in the allowed CSG list. If the identityinformation of the target femto cell base station 450 is included in theallowed CSG list, the serving base station 420 makes a decision tohandover the UE to the target femto cell base station 450 in step S510.Here, the handover decision can mean the handover initiation and accesscontrol.

In step S515, the serving base station sends a Handover Required messageto a source MME 430 located on a core network. The Handover Requiredmessage includes the identity information of the target femto cell basestation 450.

Alternatively, when the serving base station 420 does not include anallowed CSG list, step S510 is omitted, and the source MME 430 can makea handover decision when it receives the Handover Required message.

When required, the source MME 430 can send a Forward Relocation Requestmessage including the identity information of the target femto cell basestation 450 to the target MME 440 to which the target femto cell basestation 450 is connected in step S520. However, when the target femtocell base station 450 is managed by the source MME 430, there is no needto transmit a Forward Relocation message.

In step S525, the target MME 440 sends a Handover Request messageincluding the identity information of the target femto cell base station450 to the target femto cell base station 450.

At this point, the target femto cell base station 450 verifies theidentity information that is provided to the serving base station fromthe UE. More specifically, in step S530, because the identityinformation of a target femto cell base station can be changed at anytime and the identity information reported by the UE 150 is notcompletely reliable, the target femto cell base station 450 compares theidentity information (i.e., the CSG ID sent from the UE to the servingbase station) with its own identity information (i.e., its current CSGID) to determine whether they correspond.

If the identity information correspond with each other, the target femtocell base station 450 accepts the handover thereto and sends a HandoverRequest ACK message to the target MME 440 in step S535. However, if theidentity information do not correspond with each other, the target femtocell base station sends a Handover Failure message to the target MME440, notifying the UE 150 of the rejection on the handover.

In step S540, the target MME 440 sends a Forward Relocation Responsemessage to the source MME 430, in step S545, the source MME 430 sends aHandover command to the serving base station 420, and in step S550, theserving base station 420 forwards the Handover command to the UE 150.

In step S555, the UE 150 performs the handover to the target femto cellbase station 450.

Thereafter, other information related to the handover and mobilitycontrol of the UE 150 within the network can be transmitted to theserving base station 420 by means of the Measurement Report message.

FIG. 6 is a flowchart illustrating operations of a target femto cellbase station in a handover procedure as illustrated in FIG. 5.

Referring to FIG. 6, the target femto cell base station 450 receives aHandover Request message including identity information (e.g., CSG ID)of the target femto cell base station from the serving MME 430 or thetarget MME 440 in step S610. In step S620, the target femto cell basestation 450 determines whether the identity information contained in theHandover Request message corresponds with its current identityinformation. That is, the target femto cell base station 450 determineswhether the CSG ID reported from the UE 150 to the serving base stationmatches its current CSG ID.

When the received CSG ID matches with the current CSG ID, this meansthat the target femto cell base station 450 has been successfullydetected by the UE 150, and thus, the target femto cell base station 450sends a Handover Request ACK message to the corresponding MME in stepS630. However, when the received CSG ID does not match with the currentCSG ID, this means that the UE 150 has reported incorrect identityinformation to its serving base station, and thus, the target femto cellbase station 450 sends a Handover Failure message to the correspondingMME, notifying that the handover attempt has been rejected, in stepS640.

FIG. 7 is a sequence diagram illustrating operations of network entitieswhen a UE performs a handover to a femto cell base station according tothe third embodiment of the present invention. More specifically, in theembodiment illustrated in FIG. 7, information on a target femto cellbase station is not included in an NRT of a serving base station and atarget MME knows the identity information of the target femto cell basestation.

Referring to FIG. 7, the serving base station 420 does not store theinformation about the neighbor femto cell base stations in an NRT.Instead, the serving base station 420 receives an allowed CSG listincluding information about the neighbor femto cells that allow theaccess of the UE 150. Further, the target MME 440 knows the identityinformation (i.e., CSG ID) of the target femto cell base station 450.

When a handover is required, the UE 150 receives an SIB broadcast by thetarget femto cell base station 450 in step S705. The UE 150 can receivethe SIB in response to a command transmitted by the serving base station420 or according to its own decision.

The UE 150 determines whether the target femto cell base station 450 isa femto cell that allows access by the UE 150 based on the receivedidentity information (i.e., CSG ID). If the target femto cell basestation 450 is an access allowed femto cell base station, the UE 150sends a Measurement Report message including the identity information ofthe target femto cell base station 450 to the serving base station 420in step S710.

Alternatively, the identity information of the target femto cell basestation 450 can be sent by some other message than a Measurement Reportmessage.

After the Measurement Report message has been received, the serving basestation 420 checks whether the identity information of the target femtocell base station 450 is included in the allowed CSG list. If theidentity information of the target femto cell base station 450 isincluded in the allowed CSG list, the serving base station 420 makes adecision to handover the UE 150 to the target femto cell base station450 in step S715 and sends a Handover Required message to the source MME430 in step S720. The Handover Required message includes the identityinformation of the target femto cell base station 450, such as CSG ID,ECGI, and TAI.

Alternatively, when the serving vase station 420 does not include anallowed CSG list, step S715 is omitted, and the source MME 430 can makea handover decision when it receives the Handover Required message.

When required, the source MME 430 sends a Forward Relocation Requestmessage including the identity information of the target femto cell basestation 450 to the target MME 440 to which the target femto cell basestation 450 is connected in step S725. However, when the target femtocell base station 450 is managed by the source MME 430, there is no needto transmit a Forward Relocation message.

In FIG. 7, it is assumed that each femto cell base station has a singlecell and the target MME 440 has the identity information of all thefemto cell base stations.

In step S730, the target MME 440 verifies the target femto cell basestation 450 by comparing the identity information of the target femtocell base station to which the UE 150 attempts handover and the actualidentity information of the target femto cell base station 450. When twoidentity information correspond with each other, the handover procedurecontinues. However, when the two identity information do not correspondwith each other, a handover failure is notified to the UE with thehandover rejection of the target MME 440.

When it has been determined that the two identity information correspondwith each other at step S730, the target MME 440 sends a HandoverRequest message to the target femto cell base station 450 in step 735,and the target femto cell base station 450 sends a Handover Request ACKmessage notifying acceptance of the handover to the target MME 440 inresponse to the Handover Request message in step S740.

In step S745, the target MME 440 sends a Forward Relocation Responsemessage to the source MME 430, in step S750, the source MME 430 sends aHandover command to the service base station 420, and in step S755, theserving base station 420 forwards the Handover command to the UE 150.

In step S760, the UE 150 performs the handover to the target femto cellbase station 450.

Thereafter, other information related to the handover and mobilitycontrol of the UE 150 within the network can be transmitted to theserving base station 420 by means of the Measurement Report message.

FIG. 8 is a sequence diagram illustrating operations of network entitieswhen a UE performs a handover to a femto cell base station according toan embodiment of the present invention. More specifically, in theembodiment illustrated in FIG. 8, information on a target femto cellbase station is not included in an NRT of a serving base station.

Referring to FIG. 8, the serving base station 420 does not store theinformation about the neighbor femto cell base stations in an NRT, anddoes not store an allowed CSG list.

When a handover is required, the UE 150 receives an SIB broadcast by thetarget femto cell base station 450 in step S805. The UE 150 can receivethe SIB in response to a command transmitted by the serving base station420 or according to its own decision.

The UE 150 determines whether the target femto cell base station 450 isa femto cell that allows access by the UE 150 based on received identityinformation (i.e., CSG ID). If the target femto cell base station 450 isan access allowed femto cell base station, the UE 150 sends aMeasurement Report message including the identity information of thetarget femto cell base station 450, such as CSG ID, ECGI, and TAI, tothe serving base station 420 in step S810.

Alternatively, the identity information of the target femto cell basestation 450 can be sent by some other message than a Measurement Reportmessage.

After the Measurement Report message has been received, the serving basestation makes a handover decision for the UE 150 in step S815 or sends aHandover Required message to request for the source MME 430 in stepS820, and the source MME 430 makes a handover decision in step S825.

As described above, the serving base station 420 has no allowed CSG listand thus, can only determine the handover execution (i.e., handoverinitiation) and leaves the handover decision to the MME in a corenetwork.

More specifically, the serving base station 420 transmits the identityinformation of the target femto cell base station 450 to the source MME430, and in step S825, the source MME 430 checks whether the CSG ID ofthe target femto cell base station 450 is included in the allowed CSGlist of the UE 150 to make a handover decision. When the CSG ID of thetarget femto cell base station 450 is included in the allowed CSG listof the UE 150, the handover is performed, and when the CSG ID of thetarget femto cell base station 450 is not included in the allowed CSGlist of the UE 150, the handover is not performed.

When required, the source MME 430 sends a Forward Relocation Requestmessage including the identity information of the target femto cell basestation 450 to the target MME 440 to which the target femto cell basestation is connected in step S830. However, when the target femto cellbase station 450 is managed by the source MME 430, there is no need totransmit a Forward Relocation message

In step S835, the target MME 440 sends a Handover Request messageincluding the identity information of the target femto cell base station450 to the target femto cell base station 450.

Again, the target femto cell base station 450 verifies the identityinformation of the target femto cell base station 450 that is providedto the serving base station from the UE 150, because the identityinformation of the target femto cell base station 450 can be changed atany time and the identity information reported by the UE 150 is notcompletely reliable.

Accordingly, in step S840, the target femto cell base station 450compares the identity information (i.e., the CSG ID sent from the UE tothe serving base station) with its own identity information (i.e., acurrent CSG ID) to determine whether to correspond with each other. Ifthe identity information correspond with each other, the target femtocell base station 450 accepts the handover thereto and sends a HandoverRequest ACK message to the target MME 440 in step S845. However, if theidentity information do not correspond with each other, the target femtocell base station 450 sends a Handover Failure message to the target MME440, notifying the UE 150 of the rejection on the handover.

In step S850, the target MME 440 sends a Forward Relocation Responsemessage to the source MME 430, in step S855, the source MME 430 sends aHandover command to the serving base station 420, and in step S860, theserving base station 420 forwards the Handover command to the UE 150.

In step S865, the UE 150 executes the handover to the target femto cellbase station 450.

Thereafter, other information related to the handover and mobilitycontrol of the UE 150 within the network can be transmitted to theserving base station 420 using the Measurement Report message.

FIG. 9 is a block diagram illustrating a femto cell base stationsupporting a handover procedure according to an embodiment of thepresent invention.

Referring to FIG. 9, the femto cell base station includes an RF unit910, a storage unit 920, and a control unit 930. The storage unit 920includes a cell information storage region 920A, and the control unit930 includes a cell identity verifier 930A.

The RF unit 910 includes an RF transceiver and a duplexer (not shown)and is responsible for processing RF signals. The RF unit 910 receivesthe Handover Request message transmitted by the source MME 430 or thetarget MME 440. Here, the handover Request message can carry theidentity information of the target femto cell base station 450 to whichthe UE 150 attempts to access.

The storage unit 920 stores application programs and data related tooperations of the target femto cell base station 450. Further, the cellinformation storage region 920A including in the storage unit 920 storesthe cell information of the target femto cell base station. The cellinformation includes ECGI, CSG ID, and TAI of the target femto cell basestation 450.

The control unit 930 controls the overall operations of the femto cellbase station. The control unit 930 controls the RF unit 910 to receiveand process the Handover Request message transmitted by the source MME430 or the target MME 440. If the Handover Request message has beenreceived, the control unit 930 extracts the identity information of thetarget femto cell base station 450 from the Handover Request message andsends the identity information to the cell identity verifier 930A.

The cell identity verifier 930A compares the identity information (i.e.,a CSG ID) that the UE 150 reports to its serving base station 420 withcurrent identity information of the femto cell base station 450 todetermine whether they correspond with each other. If the two CSG IDscorrespond with each other, the cell identity verifier 930A accepts thehandover and controls the femto cell base station 450 to send theHandover Request ACK message to the target MME 440 or the source MME430.

However, if the two CSG IDs do not correspond with each other, the cellidentity verifier 930A sends a Handover Failure message, notifying thatthe handover of the UE 150 has been rejected.

As described above, the handover method and apparatus of the presentinvention facilitates a handover of a UE to a femto cell in a wirelesscommunication system supporting both the femto and macro cells.

Although certain embodiments of the present invention have beendescribed in detail hereinabove, it should be clearly understood thatmany variations and/or modifications of the basic inventive conceptstaught herein, which may appear to those skilled in the present art,will still fall within the spirit and scope of the present invention, asdefined in the appended claims.

What is claimed is:
 1. A handover method of a terminal in a wirelesscommunication system, the method comprising: sending a first measurementreport to a source base station, the first measurement report includinga cell identifier (ID) of a target base station; receiving controlinformation to perform system information acquisition, from the sourcebase station; receiving system information including a tracking areaidentifier (TAI) and a closed subscriber group identifier (CSG ID), fromthe target base station; sending a second measurement report includingthe TAI and the CSG ID of the target base station, to the source basestation; and receiving handover command information from the source basestation, wherein the CSG ID included in the second measurement report iscompared, by the target base station, with a CSG ID broadcast in thetarget base station to verify whether the CSG ID in the secondmeasurement report matches a CSG ID broadcast in the target basestation.
 2. The handover method of claim 1, wherein it is verifiedwhether the CSG ID in the second measurement report matches a CSG IDbeing broadcast from the target base station.
 3. The handover method ofclaim 2, wherein the CSG ID included in the second measurement report isused for access control in a mobility management entity (MME).
 4. Thehandover method of claim 1, wherein the handover command information isreceived from the source base station, when the CSG ID transmitted tothe target base station is verified.
 5. The handover method of claim 1,further comprising: initiating a handover procedure in response toreceiving the handover command information.
 6. A terminal performing ahandover in a wireless communication system, the terminal comprising: atransceiver configured to transmit and receive a signal; and acontroller configured to send a first measurement report to a sourcebase station, the first measurement report including a cell identifier(ID) of a target base station, to receive control information to performsystem information acquisition, from the source base station, to receivesystem information including a tracking area identifier (TAI) and aclosed subscriber group identifier (CSG ID), from the target basestation, to send a second measurement report including the TAI and theCSG ID of the target base station, to the source base station, and toreceive handover command information from the source base station,wherein the CSG ID included in the second measurement report iscompared, by the target base station, with a CSG ID broadcast in thetarget base station to verify whether the CSG ID in the secondmeasurement report matches a CSG ID broadcast in the target basestation.
 7. The terminal of claim 6, wherein it is verified whether theCSG ID in the second measurement report matches a CSG ID being broadcastfrom the target base station.
 8. The terminal of claim 7, wherein theCSG ID included in the second measurement report is used for accesscontrol in a mobility management entity (MME).
 9. The terminal of claim6, wherein the handover command information is received from the sourcebase station, when the CSG ID transmitted to the target base station isverified.
 10. The terminal of claim 6, wherein the controller isconfigured to initiate a handover procedure in response to receiving thehandover command information.
 11. A source base station in a wirelesscommunication system, the source base station comprising: a transceiverconfigured to transmit and receive a signal; and a controller configuredto receive a first measurement report from a terminal, the firstmeasurement report including a cell identifier (ID) of a target basestation, to send control information to perform system informationacquisition, to the terminal, to receive a second measurement reportincluding a tracking area identifier (TAI) and a closed subscriber groupidentifier (CSG ID), of the target base station, from the terminal, andto send handover command information to the terminal, wherein the CSG IDincluded in the second measurement report is compared, by the targetbase station, with a CSG ID broadcast in the target base station toverify whether the CSG ID in the second measurement report matches a CSGID broadcast in the target base station.
 12. The source base station ofclaim 11, wherein it is verified whether the CSG ID in the secondmeasurement report matches a CSG ID being broadcast from the target basestation.
 13. The source base station of claim 12, wherein the CSG IDincluded in the second measurement report is used for access control ina mobility management entity (MME).
 14. The source base station of claim11, wherein the handover command information is sent to the terminal,when the CSG ID included in the second measurement report is verified.15. The source base station of claim 11, wherein the handover commandinformation is for the terminal to initiate a handover procedure.
 16. Ahandover method of a source base station in a wireless communicationsystem, the method comprising: receiving a first measurement report froma terminal, the first measurement report including a cell identifier(ID) of a target base station; sending control information to performsystem information acquisition, to the terminal; receiving a secondmeasurement report including a tracking area identifier (TAI) and aclosed subscriber group identifier (CSG ID) of the target base station,from the terminal; and sending handover command information to theterminal, wherein the CSG ID included in the second measurement reportis compared, by the target base station, with a CSG ID broadcast in thetarget base station to verify whether the CSG ID in the secondmeasurement report matches a CSG ID broadcast in the target basestation.
 17. The handover method of claim 16, wherein it is verifiedwhether the CSG ID in the second measurement report matches a CSG IDbeing broadcast from the target base station.
 18. The handover method ofclaim 17, wherein the CSG ID included in the second measurement reportis used for access control in a mobility management entity (MME). 19.The handover method of claim 16, wherein the handover commandinformation is sent to the terminal, when the CSG ID included in thesecond measurement report is verified.
 20. The handover method of claim16, wherein the handover command information is for the terminal toinitiate a handover procedure.